The present invention relates to a method of and apparatus for intensifying the washing of pulp with various washing apparatus. The method and apparatus are particularly well applicable in connection with the so-called Drum Displacer washers, DD washers, by A. AHLSTROM CORPORATION, and also in some wash presses. Because the method and apparatus of the invention are applicable in connection with other washing devices also, different apparatus used in washing are discussed here.
Several types of different washing apparatus and methods are know from the prior art. Diffusers, drum washers and belt washers clearly differ from each other. Pulp is supplied into washing diffusers at a consistency of approx. 10%. The feeding consistency for drum and belt washers is most usually 1-3%. Suction washers, wash presses and pressurized or super-atmospheric washers are examples of drum washers used today.
A conventional suction washer comprises a wire-covered drum revolving in a vat. The shell of the drum comprises under a perforated plate collecting compartments, and each compartment is connected with a tube of its own to a valve system on the shaft at the end of the drum. Filtrate from the valve is guided via a drop leg, or a centrifugal pump providing the required suction, for example to a filtrate tank. Due to the valve arrangement, the influence of the drop leg may be directed appropriately in the desired spots of the web formation.
Web formation in a suction washer takes place as follows: inside the drum revolving in the vat, sub-atmospheric pressure sucking pulp suspension from the vat onto the surface of the drum has been arranged by means of a drop leg or some other device generating suction. When the liquid passes through the drum the fibers in the pulp are collected onto the surface of the drum. The consistency of the suspension in the drum in approx. 0.5-2% and the consistency of the layer thickened onto the drum surface is approx. 10-12%. The web formation area, i.e. the portion of the drum periphery which is in the vat in the fiber suspension, is about 140 degrees. The maximum revolution velocity of the drum is 2-2.5 r/min; at higher revolutions speeds the filtrate collecting compartments and tubes do not have time to be emptied.
Washing is carried out as displacement wash by spraying wash liquid onto the surface of the drum which has risen up from the pulp vat. The sub-atmospheric pressure sucks the wash liquid through the pulp layer and displaces most of the liquid in the pulp. Thus, the displacement area is about 120 degrees. The typical specific square load of a suction washer is approx. 5 BDMT/m.sup.2 /d and the thickness of the pulp web is of the order of 25 mm. In a bleaching plant, the square load of a suction washer is about 8 BDMT/m.sup.2 /d and the web thickness about 30 mm.
A wash press comprises a drum covered with a wire or having a drilled perforated plate shell. Pulp is fed at a consistency of 3-4% and knots and corresponding impurities must have been removed from the pulp prior to the washer. There are compartments provided in the shell of the drum from which filtrate is discharged via a chamber at an end periphery. Also, the drum may be open so that filtrate is collected inside the drum and is discharged via an opening at an end.
The length of the web formation stage is about 90 degrees and that of the displacement stage about 150 degrees. The revolution velocity of the drum is about 2 r/min and the specific square load about 15-20 BDMT/m.sup.2 /d. The consistency of the washed web may rise even up to 35%.
The displacement, however, takes place at a consistency of about 10-15% while the thickness of the pulp web is about 30-50 mm.
An example of a superatmospheric pressure washer is a device disclosed in FI patent publications 71961 and 74752, which is composed mainly of a rotating drum and a stationary shell surrounding the drum. The drum is comprises a perforated cylinder the outer surface of which is provided with 50-60 mm high ribs at about 200 mm spacing. These ribs form with the perforated cylinder surface the so-called pulp compartments. There are filtrate compartments provided inside the cylinder under the pulp compartments, into which the filtrate displaced by the wash liquid is collected. There is a valve arrangement at the end of the cylinder drum substantially at the periphery of the diameter via which valve arrangement the filtrate is discharged and transported further. The washer comprises several, usually 3-4 stages. This means that the wash liquid is reused many times for washing the pulp; thus, the filtrate collected in the filtrate compartments is guided countercurrent from one washing stage to another. Outside the washer drum, as a part of the washer shell, there are wash liquid feed chambers from which the wash liquid is pressed through the perforated plate to the pulp in the pulp compartments to displace the liquid in the pulp.
Web formation and washing of the pulp is carried out by supplying the pulp to be washed via a particular feed box to the pulp compartments. The feed box may thicken the pulp and axial "bars" of the same length as the drum are formed in the pulp compartments. Immediately after the feed point, there is the first washing zone on the drum; there are five separate washing zones in the apparatus described in the publications mentioned. A wash liquid flow is guided to each of these zones and the wash liquid, while being pressed into the pulp layer in the compartments of the washing drum, displaces the liquid in the pulp. As already mentioned above, the filtrates are guided countercurrent from one zone to another. In other words, (cf. FI patent 74752, FIG. 1) clean wash liquid is pumped into the last washing stage and the filtrate displaced by this liquid is taken to the second last washing stage to serve as wash liquid. After the last washing stage, the "pulp bars" are detached from the drum, for example by blowing with pressurized air, and transported further on a transport screw.
The typical specific square load of a pressurized washer of this type with four stages is approx. 2.4 BDMT/m.sup.2 /d. The thickness of the "pulp bar" is about 50 mm and the consistency may rise even up to 15-18%. However, wash water leaking from the compartment decreases the consistency to 10-12%. The consistency of the pulp fed onto the drum may vary between 3,5 and 10%. The drum is rotated at about 0.5-3.0 rpm.
The FI patent 74752 mentioned above (corresponding U.S. Pat. Nos. 4,919,158 and 5,116,423) and the appended FIG. 2 illustrate schematically a little more advanced version of the basic approach of FI patent 71961, by means of which a remarkably better washing result is obtainable than with the basic arrangement illustrated schematically in the appended FIG. 1. In the embodiment of FIG. 2, each washing stage has been divided into two zones so that two washing filtrates with different concentrations are obtained from each stage. These filtrates are recycled countercurrent as illustrated in the Figure. The figure illustrates also how the so-called suction filtrate, i.e. the filtrate extracted from the point between the last washing stage and the pulp discharge, is taken, with the washing filtrate from the latter washing zone of the last washing stage, to the latter washing zone of the second last washing stage to be used as wash liquid.
It is typical of all the above apparatus that at least either the feed of the wash liquid or the treatment of the filtrates or both at the same time show drawbacks. These drawbacks may result in among other things poor washing result. If a washer is found not to be able to reach an adequate washing result the consequence naturally is that a washer with more washing stages or even a washer of a different type is acquired. It may also be necessary to try to solve the problem by increasing the consumption of clean wash liquid which increases the demand of steam in the evaporation plant and the capacity of waste water treatment equipment has to be increased and partly also environmental load increases.
The object of the invention is to solve the problems described above and to introduce arrangements applicable in many different washer types by means of which washing results are achieved which are very close to the optimal washing results obtainable with each washer or process type.
The characteristic features of the method and the apparatus are disclosed in the appended patent claims.